How much coffee is too much - and why are some people immune to the caffeine jitters?

Caffeine is a fairly powerful stimulant: it makes you feel more awake and focused, but too much can leave you in an anxious spiral.

It’s the second flat white that does it. I would barely be awake without the first, which I pick up on the way into the office. But the second – which always seems like a good idea at the time – all too often leaves me jittery and overstimulated. This is the strong stuff: unlike weaker instant coffee, or tea, which contain roughly 100mg and 75mg of caffeine respectively, a high-street flat white can contain over 250mg.

We are a nation of fully-fledged coffee addicts, consuming approximately 98 million cups per day, according to the British Coffee Association. But we forget that, fundamentally, caffeine is a fairly powerful stimulant: it makes you feel more awake and focused, but too much can potentially leave you in an anxious spiral. As my colleague Bryony Gordon has written, “coffee is essentially panic juice”. Which goes to show that, clearly, if you are prone to anxiety, coffee is not the drink you should turn to to calm down.

Why does this happen? “Caffeine blocks adenosine receptors (which promote sleep), which increases the production of dopamine, noradrenaline and glutamate [neurotransmitters that play a role in cognitive function],” explains Clare Thornton-Wood, a dietician and British Dietetics Association spokesperson.

“That makes you more alert and increases your blood pressure and heart rate. In many cases, this is beneficial.” That’s why coffee boosts your mood and helps you concentrate.

But there’s a tipping point. “The negative impacts of caffeine are caused when this effect goes too far and promotes anxiety,” Thornton-Wood says.

There is no clear reason why drinking two cups of coffee seems to tip some of us over the edge, while others can easily tolerate four or five cups. Genetics play a role, and generally your caffeine tolerance is greater if you are heavier, says Thornton-Wood. “Only some people are susceptible to the effects of caffeine; if you are more anxious in any case, you’re probably more susceptible.”

Robusta coffee beans contain roughly double the caffeine of Arabica beans.

Too much caffeine leaves you “feeling restless, nervous, not being able to sleep, feeling nauseous, having a headache and general anxiety”, says Thornton-Wood. The formal classification for this group of symptoms is “caffeine-induced anxiety”, which is actually “a recognised disorder in the DSM-5 – the Diagnostic and Statistical Manual of Mental Disorders”, she says.

The good news is that research shows a coffee habit can have significant benefits for physical health. A 2021 study published in the journal PLOS Medicine found that moderate caffeine intake cuts the risk of a stroke by almost a third and the risk of dementia by a quarter. Another major study from scientists at the universities of Southampton and Edinburgh found that two cups of coffee per day reduces the risk of liver cancer by a third. That’s not all: a 2014 meta-analysis revealed that drinking up to four cups of caffeinated coffee a day was associated with a 25 per cent lower risk of developing diabetes.

Other studies show that it boosts athletic performance: in 2017, researchers from the University of São Paulo put 40 cyclists through a series of time trials and found that a caffeine supplement boosted performance by an average of 2.5 per cent compared to a placebo. These benefits are all associated with a “moderate” caffeine intake of roughly one to four coffees per day – drinking five or more can have adverse health effects. And yet, for some, drinking more than one cup (or in some cases, any at all) can wreak havoc.

There is no golden rule for when you should drink coffee in order to enjoy the buzz and skip the jitters. “It all depends on the dose of caffeine in your coffee and how close together you’re having them,” says Thornton-Wood. “It’s personal to you. I can drink coffee all day and after dinner and it doesn’t make any difference to me. I know the rest of my family won’t drink coffee after mid-afternoon. Everybody’s different.”

How much you should drink is a more complicated question. While 400mg of caffeine per day is the NHS’s recommended limit (200mg for pregnant women), the amount of caffeine in various shop-bought coffees varies wildly, which can make it difficult to keep track of your consumption.

According to data from Which?, a single cappuccino from Costa would put you close to the maximum recommended intake, with 325mg of caffeine (more than double that of a Starbucks one, and almost as much as four cans of Red Bull). However, you could drink three cappuccinos from Café Nero, at 120mg of caffeine each, and still be under the limit.

The variations are not only down to how many shots of coffee each chain uses, but also on the coffee beans themselves: of the two most commonly used, Robusta beans contain roughly double the caffeine of Arabica beans. So you may find you feel the negative effects of too much caffeine after two flat whites from a coffee shop, but not after four weaker instant coffees, or after a latte from one chain but not from another.

How can you beat the two-coffee tipping point? Know your limits: if you often find yourself in a cappuccino-induced spiral, cut down and stick to one caffeinated coffee, one decaf, which has some of the health benefits of caffeinated coffee with none of the stimulating effect.

Caffeine 101

How much caffeine should you drink per day?

The safe limit is generally considered to be 400mg of caffeine per day. The NHS states that the limit is 200mg for pregnant women.

Should you hydrate before you caffeinate?

Caffeine is a diuretic compound which increases the production of urine and can, in large volumes, have a dehydrating effect. It is a good idea to alternate glasses of water with coffees to stay hydrated (although the NHS recommendation of 6-8 glasses of fluid per day includes tea and coffee, so caffeinated beverages still count).

Should you eat before you drink coffee, or with a coffee?

Drinking a coffee with full-fat milk might slightly slow its effects. There is little evidence that eating food with coffee boosts its performance-enhancing benefits.

Are two home-made cups of tea the equivalent to one shop-bought coffee?

It is quite difficult to say exactly how much caffeine is contained in a shop-bought coffee as it varies significantly by brand. A cup of tea contains around 75mg of caffeine – roughly half of that in a standard mug of filter coffee.